It seems, roughly, that “SIOCDELRT: No such device” happens when the command can’t identify which route you are talking about. A situation where it does know the route but you are trying to do something nonsensical with it seems to result in the equally unhelpful “SIOCADDRT: No such process”. For example…

It happens because the chosen gateway 10.214.49.150 is not not routable from any interface on the local machine (either eth4 or eth5). Try again with a different gateway:

[root@jim-redhat65 ~]# route add 172.27.99.11/32 gw 192.168.237.150

That works, because 192.168.237.150, (it actually doesn’t exist) is routable as an IP address. It can be contacted from eth5, which is on the same vlan. Therefore the desired route makes sense, and the kernel is happy to add it.

Acknowledgements

Thanks to the ever reliable Softpanarama web pages for the route del command. (See under the Typical Operations section). The same page also has a nice introduction to policy based routing.